Sediments occurring in many of the world's aquatic environments, including harbors and shipyard areas, are impacted by a variety of organic and/or heavy metal contaminants. Organic contaminants can include TBT, dioxins, PCBs, PAHs and/or other petroleum products whereas heavy metals can include Pb, Cu, Cr, Cd, Hg and others. In many locations, concentrations of one or more of these sediment contaminants can pose unacceptable health risks to humans and/or ecological receptors. The most direct approach for effectively lowering these risks to acceptable levels is to remediate the offending sediment contaminants.
A variety of in place (in situ) and remote (ex situ) methods exist for remediating (managing) contaminated sediments, including removal, treatment, capping and natural recovery. The most appropriate method(s) for use at a given site will vary, depending on site and sediment conditions, remediation goals, costs and other factors.
In situ approaches for managing contaminated sediments and the risks they pose—in situ capping and in situ treatment, in particular—are rapidly gaining national and international favor. Increased positive recognition of in situ capping and treatment is likely due to the advantages of these two approaches (relatively lower cost, lower environmental impact during implementation and ability to rapidly and significantly reduce risks), in combination with the recognized drawbacks to other management approaches (e.g. high costs associated with removal, dredging residuals, and slow rates associated with natural recovery).
In situ capping involves covering contaminated sediment in place with one or more clean materials. In situ capping is typically conducted for the purpose of providing a barrier between sediment-borne contaminants and potential ecological and/or human receptors occurring in the overlying aquatic ecosystem, including benthic organisms living in bottom substrates. Materials used to cap contaminated sediments can be either inert or chemically/biologically reactive in character, and often comprise earthen materials, engineered materials or combinations thereof. Cap designs can range from relatively simple (e.g. a “monolayer” of a single material, like natural, quartz-rich sand) to relatively complex (e.g. a “composite cap” comprised of multiple materials used in various configurations). The barrier formed by the cap can intentionally be relatively permeable or impermeable in character, depending on attributes of materials included in the design. Furthermore, the contaminated sediment to be capped may comprise naturally deposited sediment, dredged sediment re-deposited in a new underwater location or combinations thereof.
In situ treatment involves physically incorporating one or more reactive materials directly into the contaminated sediment mass for the purpose of stimulating or enhancing biological and/or chemical processes that bring about an in-place reduction in contaminant mass; toxicity, solubility and/or mobility.
Active sediment capping involves covering contaminated sediment with chemically and/or biologically reactive material for the purpose of treating sediment-borne contaminants. Like in situ treatment, contaminant treatment within the context of active capping generally involves bringing about a reduction in contaminant mass, toxicity, solubility and/or mobility. However, unlike in situ treatment, the “zone of treatment” for active capping occurs within the capping layer itself rather than within the underlying sediment mass. That is, in order for active capping layers to be effective, sediment-borne contaminants must first migrate up into the active capping layer, in one or more forms (i.e. dissolved phases and/or particle-bound phases). Little to no “passive” contaminant treatment is typically expected to occur within the sediment mass beneath active caps.
The specific process or processes by which contaminant treatment occurs through active capping (e.g. biodegradation, decreased solubility due to increased contaminant sorption or exchange to reactive organic or mineral solid phases, and the like) depends on the type of reactive material included as well as the mobilized contaminant(s) targeted for treatment.